The muscle mass on the pinky side of your hand is responsible for pulling that side of your hand up into an arch. Try cupping your hand like you are going to hold water in it and watch that muscle mass bunch up, giving the skin a puckered appearance. Weight-bearing on your palms as an infant helped develop that muscle mass. Some children who didn't spend a lot of time on their open palms (and even some who did) need some help developing that muscle. Often, children with Down's Syndrome have underdeveloped palmar arches. By massaging the muscle there, you can actually encourage it to develop. Here's how:

Place your thumb right in the middle of your opposite hand's palm. Now squeeze the palm pretty tightly and slide your thumb towards the pinky side of the hand, aiming for just about half-way down your palm. Mary Benbow, OTR, tells kids who need to do this to "do it anytime you're standing around waiting and you think of it." You can't really overdo it. Try it now about 15 times and then see if you can see a difference in the puckering of the skin on that side of your hand as the muscles pull your hand into a cup.